This invention relates to the art of conditioning indoor living, working and other enclosed public spaces. More particularly, this invention relates to a system in which the carbon dioxide (CO2) level is monitored and controlled by apparatus in which the CO2 sensor and support circuitry is integral with a thermostat which also serves to conventionally control the temperature range within the conditioned space.
The ever increasing cost of energy has resulted, in recent years, in the construction of homes and buildings which are very well insulated and, further, which carefully provide for keeping the air within the home or building well isolated from the encroachment of outdoor air. This latter feature serves to limit the adverse effect of outdoor air on the desired temperature intended to be maintained within the building occasioned by direct mixing with the indoor air, an effect which is in addition to and independent of the limitation of adverse conduction effects afforded by insulating the conditioned spaces.
However, it has been observed that these highly-efficient, substantially closed, environmental conditioning systems have a serious drawback which can even reach dangerous levels. Because of the natural breathing processes of the occupants of a closed conditioned space and, in some instances, because of the effects of combustion or similar processes, the concentration level of oxygen (O2) decreases while the concentration of carbon dioxide (CO2) increases. It has been found that when the concentration of CO2 in a conditioned space reaches on the order of 1000 parts per million (0.1%), breathing becomes noticeably more labored and difficult, particularly for those with breathing problems. Higher concentrations exacerbate these problems and can reach dangerous levels.
Accordingly, recommended maximum allowable concentrations of CO2 for living and working spaces have been formulated and promulgated, and, in some instances, various government agencies have imposed requirements that specify the maximum concentration of CO2 which will be allowed in public conditioned spaces.
Commercial CO2 detectors have been not only bulky, but have also provided measurements of CO2 concentration only across a very narrow, low level range. One commonly used CO2 sensor in CO2 detectors constitutes an IR light source directed at an IR light sensor such that the CO2 concentration between the source and sensor subtly affects the response of the source. As a result, the support circuitry has to be able to interpret a suitably compensated signal (for example, for temperature and/or humidity variations) which may change only by a few microvolts across the entire measurement range.
A prior art approach to solving these problems is disclosed and claimed in U.S. patent application Ser. No. 09/240,506, filed Jan. 29, 1999, by Howard B. Rosen and Steven D. Dushane for Thermostat Incorporating Thin Film Carbon Dioxide Sensor and Environmental Control System, now U.S. Pat. No. 6,398,118, dated Jun. 4, 2002. In this reference, the CO2 sensor consists of a cathode disposed on a substrate, an anode disposed on the substrate spaced from the cathode and a solid electrolyte disposed on the substrate intermediate and electrically in contact with each of the cathode electrode and the anode to effect a primary electrical cell. A heater and a heater thermostat are also disposed on the substrate and are connected to a source of electrical energy to maintain the primary electrical cell within a predetermined temperature on the order of 250xc2x0 C. The metals from which the cathode and anode are fabricated and the chemical composition of the electrolyte were selected, as discussed in detail in the reference, such that the voltage established between the cathode and the anode varies in accordance with the CO2 concentration at the primary electrical cell when the temperature of the cell is within the predetermined temperature range.
It has been found that the CO2 sensor discussed in the above-identified reference has certain practical drawbacks when incorporated into a commercial thermostat. One significant practical problem is associated with the need to maintain the cell within the relatively high predetermined temperature range for both the short term (power consumption and heat stress on associated circuitry, particularly in a single chip embodiment) and the long term (operational life and creeping deterioration). Additional practical drawbacks include the relatively high cost of fabrication, limited robustness and generally limited long term reliability.
It is to overcoming these drawbacks of known space conditioning control systems which incorporate a CO2 concentration control feature that the present invention is directed.
It is therefore a broad object of this invention to provide an improved CO2 detector which is sufficiently compact and simple as to be suitable for integration with a thermostat in a single housing.
It is a more specific object of this invention, in a presently preferred embodiment, to incorporate into a thermostat a small, inexpensive and accurate CO2 sensor which, if a heater is used, requires significantly less power to heat than the CO2 sensor disclosed and claimed in the above identified reference and is more reliable in both the long and short terms.
Briefly, these and other features of the invention are found in a system for monitoring and modifying the quality and temperature of air within a conditioned space, which system includes a blower unit, an optional damper unit for selectively admitting outside air into the conditioned space, a temperature moderating unit and a control unit. The control unit includes a thermostat and conventional temperature control apparatus for selectively activating the temperature moderating unit to maintain the desired temperature in the conditioned space. The control unit also incorporates CO2 concentration measuring and control apparatus which employees a small CO2 sensor. The CO2 sensor includes upper and lower electrodes with an intermediate solid electrolyte. A voltage having a cyclic waveform is applied across the upper and lower electrodes, and the current through the CO2 sensor is read at at least one predetermined voltage representing a CO2 peak. The measured current at the predetermined voltage is representative of the CO2 concentration. CO2 concentration modifying apparatus is responsive to sensing a first predetermined CO2 concentration for turning on the blower unit and, optionally, to sensing a second, higher, predetermined CO2 concentration for actuating the damper unit to admit outside air.